The invention relates to metal-based gradient composite material having good lubrication and wear-resistance properties, the invention also relates to a process for producing the gradient composite material as well as the use of the same.
According to the structure, material can be divided into two categories, i.e. integral materials and composite materials, so is materials having lubrication and wear resistance properties. Integral materials refer to material whose structure is homogenous throughout the whole material, such as metals, plastics and ceramics. For example, sliding bearings can be made from homogenous copper alloys or plastics. There are many kinds of composite materials, some of them are exemplified as follows:
(1) Composite material comprising a matrix on which a layer of metal or polymer is coated, e.g. steel on which a layer of copper or aluminum is coated can be used as sliding bearings in the engines, and babbit metal can be further plated on the outer copper or aluminum layer.
(2) Low friction composite material comprising graphite fiber and epoxy (see U.S. Pat. No. 4,072,084).
(3) A layer for lowering friction produced by means of deposition, sputtering, electroplating, ion-plating, ion implantation and the like, e.g. Li Wenmei et al. reported their research on wear of vapor deposited TiN and Ti(CN) at elevated temperature (TRIBOLOGY; Vol.14, No.3, P205), Luo Hong et al. studied the frictional characteristic of sputtered MoS2 under grease lubrication ( TRIBOLOGY; Vol.14, No. 4, P314).
The common features of the three kinds of composite materials mentioned above lie in that they are mixtures or complex of different components, i.e. the distribution of the concentration of each component in the material is homogeneous or suddenly changed, and not gradually changed. This results in a poor comprehensive mechanical performance. When the surface layer of a composite material is selected from materials having low frictional coefficient and good wear-resistance properties, and the matrix of the composite material is selected from materials having good mechanical properties, some advantages may be obtained. But the interface between the surface and matrix is easily to peel off or rupture when the material is running under high load, because the bonding strength between the surface and matrix of the composite is low due to the mechanical bonding.
In 1987, Zhang Yifei (one of the inventors of the present application) has disclosed a process and equipment to form a sulfide case at the surface of metal parts (see CN85106828A or EP0218916). A layer of sulfide with a thickness of up to 120 xcexcm can be produced by said process, and the bonding strength between sulfide layer and metal matrix is much improved compared with the prior art materials. However, a stable sulfide gradient layer is difficult to obtain due to its sensitivity to oxygen.
After intensive research work on the field, the inventors of the invention have accidentally discovered that if a complex of metal oxide and metal sulfide gradient layer is formed on the surface of a metal matrix, the existing technical defect as mentioned above can be overcome. Further effort lead to an easy production of the novel composite gradient layer of metal sulfide and metal oxide which has a good bonding strength with the metal matrix and exhibits excellent lubrication and wear-resistance properties.
According to the first aspect of the invention, the invention provides a metal-based gradient composite material having good lubrication and wear-resistance properties, characterized in that it comprising a metal (M) matrix, and a gradient composite layer of metal sulfide (M[S]) and metal oxide (M[O]) on the surface of said metal matrix, the sum (Ds+Do) of the concentration of metal sulfide (Ds) and the concentration of metal oxide (Do) in the gradient composite layer decreases gradually from the surface to the interior, while the concentration of the metal (DM) in the gradient composite layer increases gradually from the surface to the interior.
According to a preferred embodiment of the invention, in the above-mentioned metal-based gradient composite material, both the concentration of metal sulfide (Ds) and the concentration of metal oxide (Do) in the gradient composite layer decrease gradually from the surface to the interior.
According to a preferred embodiment of the invention, in the above-mentioned metal-based gradient composite materials, said metal (M) is selected from iron (Fe), aluminum (Al), copper (Cu), nickel (Ni), molybdenum (Mo), titanium (Ti) and alloys thereof.
According to a preferred embodiment of the invention, in the above-mentioned metal-based gradient composite materials, said metal (M) matrix is selected from plating layer, coating layer, infiltration layer and deposition layer of iron (Fe), aluminum (Al), copper (Cu), nickel (Ni), molybdenum (Mo), titanium (Ti) and their alloys.
According to a preferred embodiment of the invention, in the above-mentioned metal-based gradient composite materials, the concentration of metal sulfide (Ds) and the concentration of metal oxide (Do) in said gradient composite layer satisfy the relationship of:
0% less than Do/Do+Ds less than 30% or
0% less than Ds/Do+Ds less than 30%.
According to a preferred embodiment of the invention, in the above-mentioned metal-based gradient composite material, only metal sulfide M[S] and metal oxide M[O] are existing at the outermost surface of the gradient composite layer.
According to another aspect of the invention, the invention provides a process for producing the above-mentioned metal-based gradient composite material, comprising the steps of: an anode and cathode are provided in a vacuum chamber, a metal (M) matrix is laid on the cathode after the surface of-the metal matrix has been cleaned; then sulfur and/or oxygen atmosphere is created in the vacuum chamber, and 320V-1500V of D.C. voltage or D.C. pulse voltage is applied between the anode and cathode so that glow discharge occurs, and the temperature of the metal (M) matrix increases gradually and is maintained at a temperature of 130xc2x0 C.-450xc2x0 C. for a period of 3-15 hours.
The invention process can be carried out by means of the following embodiments:
1. Vapor of sulfur was introduced into the vacuum chamber firstly, the gradient compound of metal sulfide (M[S]) was produced on metal (M) matrix through the reaction of sulfur with metal (M) under high electric voltage; then the vapor of sulfur was drawn off and oxygen was introduced into the chamber, the gradient composite material comprising metal sulfide and metal oxide (M[S]+M[O]) can be produced on metal (M) matrix at appropriate temperature.
2. Oxygen was put into the vacuum chamber firstly, the gradient metal oxide (M[O]) was produced on the metal matrix by reacting oxygen with metal (M) under high voltage; then oxygen was removed and sulfur vapor was put into the vacuum chamber, the gradient composite material comprising of metal sulfide and metal oxide (M[S]+M[O]) can be produced on metal (M) matrix at a suitable temperature.
3. Vapor of sulfur was put into the vacuum chamber firstly, the gradient metal sulfide (M[S]) was produced on metal matrix by reacting sulfur with metal (M) under high voltage; then only part of sulfur vapor was expelled from the chamber and a certain proportion of oxygen was introduced into the chamber, the gradient composite material comprising metal sulfide and metal oxide (M[S]+M[O]) can be produced on metal (M) matrix.
4. Oxygen was put into vacuum chamber firstly, the gradient metal oxide (M[O]) was produced on metal matrix by reacting oxygen with metal (M) under high voltage, then a part of oxygen was removed and a certain proportion of sulfur vapor was introduced into the chamber, the gradient composite material comprising metal sulfide and metal oxide (M[S]+M[O]) can be produced on metal (M) matrix.
5. The introduction of oxygen and sulfur atmosphere can be carried out simultaneous, alternatively or discontinuously, and the D.C. voltage or the D.C. pulse voltage can be applied continuously or discontinuously. Different concentration of gradient composite material of metal sulfide and metal oxide (M[S]+M[O]) are acquired through the reaction of metal (M) matrix with different concentration of oxygen and sulfur in the vacuum chamber at different time.
According to the requirement as designed, the formation of M[S] and M[O] may be accelerated by means of discontinuous application of D.C. voltage or D.C. pulse voltage, or introducing metal halide at a certain phase of the above process. The metal-based gradient composite material having good lubrication wear-resistance properties can be produced alternatively by the method of spray coating, deposition, powder sintering etc. The gradient composite material having required properties with different ratio of oxide and sulfide on matrix can also be produced by corporately or alternatively operation of the above means.
The metal-based gradient composite material of the invention can be produced into various shapes. They can be used to manufacture various kinds of machinery parts, such as axles, crankaxles, piston rings, valves, pump bodies, cylinders, gears, worms and worm-gears, chains, drills, hobbings, milling cutters, rolling rollers, bearing parts, and connecting pieces. Compared with the prior art composite materials comprising sputtering coating, infiltration coating or plating coating, the metal-based gradient composite material of the invention possesses good stability, high bonding strength, excellent lubrication and wear-resistance properties and favorable fatigue strength.